I think you've nailed it. I'd be really shocked to find someone working 9am-9pm straight, no lunch or dinner break 5 days a week. I used to do shift work (IT) 7am-7pm 3 days a week, which, to me, was burn out level. I used to feel over worked even though every weekend was a 4 day weekend. Can't imagine adding 2 more days.

I think that's what the author meant when he said "You become far less productive."

I can only draw from anecdotes, but those align with your suspicion: where overtime was mandatory (where management was largely about maximizing suffering ^1, as if suffering has any value by itself), everyone simply compensated by doing less real work per work hour. It does seem like many people have a fixed quantity of productive output, and the decision is simply about how much to spread it out.

This isn't true for all people. I've had weeks where I've worked 100+ hours productively because I was working on my own projects and initiatives, for my own company and enrichment, and had hit a stride. But I would never brag about that effort, because the effort itself is meaningless. Only the results of the effort mattered, and of those I am very proud. Too many brag about the effort regardless of what is often a lack of results.

^1 - http://dennisforbes.ca/index.php/2013/03/05/mediocre-manager...